1. Field of the Invention
The invention relates generally to emission control processes and devices for treating exhaust gasses produced by industrial hydrocarbon fuel burning processes involving the burning or other industrial processes evolving hydrocarbon and more particularly to processes for afterburning of exhaust gasses, afterburning systems, and afterburning devices for the reduction of NOX emissions.
2. Background Art
Industrial combustion exhaust gas emissions, and particularly emissions from automotive fuel burning and industrial fuel burning, include quantities of various undesirable chemical components including nitrogen oxides such as NO, NO2, NO3, and . . . NOX. Nitrogen oxides, commonly referred to as NOX or NOX, are a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas.
Nitrogen oxides form when fuel is burned at high temperatures, as in a hydrocarbon fuel and air combustion process. The primary manmade sources of NOX are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. NOX can also be formed naturally. In the US the Environmental Protection Agency (EPA) tracks emissions of NOX along with five other principal air pollutants carbon monoxide, lead, particulate matter, sulfur dioxide, and volatile organic compounds. In other industrialized countries NOX is also tracked, for example, emissions reporting under the Integrated Pollution Prevention and Control (IPPC) Directive and the European Pollutant Emission Register (EPER).
NOX causes a wide variety of health and environmental impacts because of various compounds and derivatives in the family of nitrogen oxides, including nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and nitric oxide. Examples of such adverse conditions include ground level ozone or smog that is formed when NOX and volatile organic compounds (VOCs) react in the presence of sunlight. NOX and sulfur dioxide react with other substances in the air to form acids which fall to earth as rain, fog, snow or dry particles (all known gas acid rain). NOX reacts with ammonia, moisture, and other compounds to form nitric acid and related particles. Human health concerns include effects on breathing and the respiratory system, and damage to lung tissue when small particles penetrate deeply into sensitive parts of the lungs and cause or worsen respiratory disease such as emphysema and bronchitis, and aggravate existing heart disease. Nitrate particles and nitrogen dioxide can also block the transmission of light and thereby reduce visibility. Increased nitrogen loading in water bodies, particularly coastal estuaries, causing water quality deterioration by upsetting the chemical balance of nutrients used by aquatic plants and animals. NOX is also considered to contribute to global warming because one member of the NOX, nitrous oxide, is a greenhouse gas that is thought to accumulate in the atmosphere with other greenhouse gasses to cause a gradual rise in the earth's temperature. In the air, NOX reacts readily with common organic chemicals and even ozone, to form a wide variety of potentially toxic chemicals such as the nitrate radical, nitroarenes, and nitrosamines.
One of the ways governmental agencies, such as the EPA in the U.S. have attempted to reduce emissions deemed to be toxic, harmful, or undesirable, is by mandating and monitoring the quality of exhaust emissions. In many instances the governmental regulations indicate that the best known process for reducing the pollutants should be used and that the amount of emissions tolerated for any given producer will not exceed a specified amount. Industry has worked to establish processes designed to reduce or eliminate such substances from the exhaust streams of industrial fuel burning processes. The mechanism for accomplishing reduced emissions can be by reducing the production of unwanted substances during burning, by providing an afterburning process for the exhaust from the primary burning process, and/or by chemically extracting or scrubbing the substances found in the exhaust stream. In the case of producing heat with a controlled fuel such as the burning of methane, low NOX burners have been developed. These devices use controlled content clean burning natural gas (methane H3C). In some low NOX burners there are two stages of burning. For example, one stage has a carefully controlled mixture of methane and air to produce a fuel rich high temperature flame and a second stage with additional air injected into the flame in sufficient amounts to both reduce the temperature and to provide complete combustion of any remaining methane fuel. These controlled methane fuel burners can be adequately controlled to significantly reduce production of either chemical NOX or thermal NOX In other processes where methane is not the only hydrocarbon compound that is evolved or burned, the initial heat might be provided by a low NOX burner, however once the other uncontrolled hydrocarbon fuel begins combustion the production of NOX was no longer controlled.
The traditional wisdom for the reduction of pollution, prior to the present invention, was to provide an after burner or a scrubber in the exhaust stream. In the case of an after burner, it was operated at a very high temperature typically above about 870° C.-950° C. For example, the EPA requirements for after burning of emissions from a sweat furnace require a residence time of 0.8 seconds and a temperature above 870° C. and EPA requirements for sludge incineration requires temperatures of about 925° C. (See example article found at www.epa.gov/ttn/atw/alum2nd/secalum.pdf.) The EPA requirements also provide for excess amounts of air, for example, in the case of sludge incinerations as much as 50%-100% excess air. The intent is to provide a combustion oxidation reaction with an amount of air to exceed the stoichiometric ratio for complete and rapid combustion so that all the hydrocarbon compounds remaining in the exhaust after the initial burning are completely burned in the after burner.
Thus, in the case of burning of hydrocarbon fuels, one primary consideration has been to attempt to burn all of the hydrocarbons as completely as possible. In most industrial burning processes, combustion is not always as complete as desired. The burning temperature and the time of burning are often dictated by the process and the fuel available for the process whether for heat generation for a particular industrial process, for steam production and electrical energy generation, for manufacturing such as concrete production in industrial steel furnaces or for other industrial processes. In many processes such as steam generation and concrete clinker production, low NOX burners are used at the initial stages to raise the temperature of other hydrocarbon fuel such as coal, heating oil, or other hydrocarbon fuel up to ignition temperature and to ignite and maintain the burning of the primary source of hydrocarbon fuel. Even where low NOX burners are used in the initial ignition and combustion maintaining part of a process, without producing excessive NOX, It has been found that while the low NOX methane burners have a burner flame that is controlled to produce little NOX, the remainder of the heated and them ignited hydrocarbon generally produces significant amounts of NOX. The hydrocarbon burning processes produce unwanted substances in the exhaust stream. Those substances must be adequately reduced in quantity in an after burning process and/or scrubbed from the exhaust stream using a high temperature catalytic scrubbing process.
The currently established standards, for industrial afterburners used on hydrocarbon burning exhaust, generally indicate burning remaining hydrocarbon fuels at temperature over about 950° C.-1000° C. in an oxygen rich atmosphere. Many facilities that attempt to comply with such standards for industrial combustion and after burner combustion processes seek to provide combustion at temperatures that exceed 1000° C. At such high burning temperatures all of the largest hydrocarbon chains typically found in commercial hydrocarbon fuels break down into lighter, volatile hydrocarbon chains that facilitate complete combustion. Complete combustion is insured by burning the exhaust gasses with excess oxidant for a sufficient period of time before finally discharging the exhaust gasses. Thus, the established best available processes for burning hydrocarbon with O2, require an excess amount of quantity of O2 and a sufficient time to provide complete combustion of all of the hydrocarbon components from the fuel. Generally, oxygen:fuel ratios of greater than 1.0:1.0 for example 1.1:1.0 are considered desirable to insure complete combustion/oxidation of all carbon (Cx) and hydrogen (Hy) in the fuel. Typically, and most economically, industrial combustion occurs in an atmosphere of air and is provided together with the air at a ration of 21(O2):79(N2). For purposes of discussion, the ratio of O2:N2 will be approximated herein as a ratio of about 2:8 and it will be understood by those skilled in the art that the actual ratio in air is closer to 2.1:7.9. Complete combustion of hydrocarbon fuel generally means that CH4 will combine with 2O2 to form CO2 and 2H2O.CH4+2O2→CO2+2H2O  (1)
In order to have an adequate supply of O2 for complete combustion with one mole of CH4, the quantity of the air will need to have 2 moles O2. Generally speaking, and according to the approximation discussed above, there are 2 moles of O2 and 8 moles of N2 in any standard quantity of 10 moles of air (2O2+8N2=10Air). Thus, when burning in air, an equivalency ratio (ER) of greater than 1 (ER>1.0) requires more than about 10 moles of air for 1 mole of CH4 (for example 10.1Air:1.0Cx). A generalized chemical equation (assuming air has an approximate ratio of 2O2:8N2) for combustion of CH4 may be expressed as follows in equation (2) below:1 Mole CH4+10 Moles AIR (2O2 and 8N2)→CO2+2H2O+8N2  (2)
When there is complete combustion of hydrocarbon with excess air at high temperatures two kinds of NOX are formed. Chemical NOX results from O2 combining with N from the hydrocarbon to form chemical NOX. It has been found by the inventors that the amount of chemical NOX is not normally significant and in most cases, if all the nitrogen from the fuel combines to form chemical NOX, that amount of NOX (i.e., the maximum possible amount of chemical NOX) would not by itself exceed the minimum permissible standards for air quality pursuant to U.S. EPA regulations and most governmental requirements throughout the world. However, when there is excess air (O2 and N2) at high burning temperatures for a sufficient time, significant and troublesome quantities of thermal NOX forms. Thermal NOX results from O2→2O and N2→2N at the high temperature and then N and (X) times O combining to form thermal NOX. The formation of thermal NOX increases exponentially (a second order parabolic function) with increase in temperature. Chemical NOX is not a major problem and the amount of chemical NOX can be controlled further by appropriately controlling the composition of the fuel, for example by using hydrocarbons having reduced nitrogen components. In burning processes where air is the oxidant, thermal NOX formation is a big concern. The high temperature burning process that controls unburned hydrocarbon emissions simultaneously promotes the formation of thermal NOX and a scrubbing unit is typically required in the exhaust gas stream to remove NOX to an acceptable level. Thus, prior to the present invention, scrubbing of exhaust gasses to remove the NOX has been a costly necessity for most fuel burning industries.